The present invention relates to an improved process for making silicone hydrogel lenses.
Hydrogels represent a desirable class of materials for contact lenses. A hydrogel is a hydrated cross-linked polymeric system that contains water in an equilibrium state. Hydrogel lenses offer relatively high oxygen permeability as well as desirable biocompatibility and comfort.
Conventional hydrogel lenses are prepared from monomeric mixtures predominantly containing hydrophilic monomers such as 2-hydroxyethyl methacrylate or N-vinyl pyrrolidone. Oxygen permeability of these conventional hydrogel materials relates to the water content of the materials. More recently, it has been desired to increase the oxygen permeability of hydrogel lenses by employing silicone-containing monomers in the hydrogel formulations since silicone based materials have higher oxygen permeability than water. Silicone hydrogels (i.e., hydrogels containing silicone) are usually prepared by polymerizing a mixture containing at least one silicone-containing monomer and at least one hydrophilic monomer. Either the silicone-containing monomer or the hydrophilic monomer may function as a crosslinking agent (a crosslinker being defined as a monomer having multiple polymerizable functionalities) or a separate crosslinker may be employed.
Various processes are known for curing a monomeric mixture in the production of contact lenses, including spincasting and static casting. Spincasting methods are disclosed in U.S. Pat. Nos. 3,408,429 and 3,660,545, and static casting methods are disclosed in U.S. Pat. Nos. 4,113,224 and 4,197,266. Curing of the monomeric mixture is often followed by a machining operation in order to provide a contact lens having a desired final configuration.
According to several known techniques for manufacturing contact lenses, the casting process may yield a shaped article having the desired posterior and anterior lens surfaces. For example, in static casting processes, a monomeric mixture can be charged to a mold comprising a first mold section including a surface for forming a desired anterior lens surface and a second mold section including a surface for forming a desired posterior lens surface. In spincasting processes, the monomeric mixture can be charged to an open mold having a surface for forming a desired anterior lens surface, and a desired posterior lens surface is formed from rotation of the mold. However, machining operations, subsequent to the curing of the article, may still be necessary to provide a contact lens more suitable for placement on the eye. Such machining operations include lathe cutting the lens to obtain a desired edge, buffing the lens edge or polishing the lens edge or surface.
In other known manufacturing techniques, the casting process may yield a shaped article which does not have the desired anterior and/or posterior lens surfaces. Accordingly, the casting process is followed by a machining operation to form a desired lens surface. As an example, U.S. Pat. No. 4,555,732 discloses a process where an excess of a monomeric mixture is cured by spincasting in a mold to form a shaped article having an anterior lens surface and a relatively large thickness, and the posterior surface of the cured spincast article is subsequently lathe cut to provide a contact lens having the desired thickness and posterior lens surface. Further machining operations may follow the lathe cutting of the lens surface, such as the previously described edge finishing operations.
However, difficulties have been encountered in attempts to employ such methods which involve machining operations in the production of silicone hydrogel contact lenses. For example, silicone hydrogel articles generally are more difficult to machine than conventional hydrogel articles, as hydrogels prepared from silicone-containing monomers tend to be softer and more rubbery than hydrogels prepared from only the conventional monomers, especially when cast with a diluent.